Part Five: The Privileged Planet

The fine-tuning of universal constants permits a universe with chemistry and stars. The fine-tuning of our solar system's configuration permits a planet with stable conditions for complex life. The entries in this section document the astronomical and geophysical circumstances that make Earth a platform for life—circumstances that compound upon each other and together represent an improbability that the science has increasingly been forced to acknowledge.

Chapter 37 — The Habitable Zone

The circumstellar habitable zone is the range of orbital distances from a star at which liquid water can exist on a planet's surface. For the Sun this zone occupies a relatively narrow band. Earth's orbit sits within it. Venus, approximately 28 percent closer to the Sun, has a surface temperature hot enough to melt lead. Mars, farther out, has a mean surface temperature of -63°C and no persistent liquid surface water.

The continuously habitable zone—the region where a planet can remain habitable over billions of years as its star gradually brightens—is narrower still. Estimates suggest it represents roughly 5 to 10 percent of the distance range around a Sun-like star. Earth not only sits in this zone—it has done so for approximately 4 billion years, during which the Sun has brightened by approximately 30 percent. Earth's surface temperature has remained stable enough for liquid water throughout, requiring geological feedback mechanisms involving the carbon-silicate cycle that operate on million-year timescales.

The stability of Earth's surface temperature over billions of years despite a brightening star is not guaranteed by the existence of a habitable zone alone. It requires the additional fortunate circumstance of a feedback mechanism that compensates for increasing solar output. Earth has it. This is not the only such circumstance that Part Five describes.

Kasting, J.F., Whitmire, D.P., Reynolds, R.T. "Habitable Zones around Main Sequence Stars." Icarus 101 (1993): 108–128. Gonzalez, G. and Richards, J.W. The Privileged Planet. Regnery Publishing, 2004.